For quite some time various foods, such as potato chips, pretzels and the like, have been packaged in flexible plastic packaging materials. Throughout this description when we mention plastics we mean to include foils, or laminated polymers and foils, or in general the materials which are used with form filled seal packaging machines to package potato chips, pretzels, nuts and the like. The general practice is to have a roll of such plastic material loaded onto a holder means and to have the plastic material drawn therefrom. The plastic material is pulled over a series of idlers, passed around a forming collar (or some means to keep the inside of the package open so that the items or food to be held by the plastic bag can be loaded into the bag), pulled through the forming collar, formed into a sealed seam along at least one side, and crimped at the top and bottom. The seam along the side of the package is usually either an overlap seal or a fin seal. The plastic material is usually pulled by crimping jaws which crimp the bottom of one package and the top of the preceding (i.e. preceding through the machine) package at the same time. The crimping jaws are housed in a mechanism which moves toward and away from the forming collar (up and down in the case of a vertical form filled seal packaging machine). There is also included a means for cutting the plastic material between the bottom seam of one package and the top seam of the preceding package.
The side seam is effected by having a heat seal bar located in close proximity to the path of the plastic which has passed through the forming collar. The heat seal bar is ideally as long as the package to be formed. As its name implies, the heat seal bar is a rigid element which is heated by an electrical heating element. As the plastic material leaves the forming collar, the material is dragged between a tongue (a rigid member located inside of the package) and the heat seal bar. The plastic material is heated all along the excursion while it is in contact with the heat seal bar. Ideally the plastic material is heated uniformly to a temperature of about 250.degree. F. to 300.degree. F. (depending on the material) and at this temperature the layers of the plastic material which are disposed in either a fin seal or an overlap seal, are joined together to form a sealed seam.
The foregoing described technique and machines have worked satisfactorily for the most part but with the advent of more stringent packaging demands thicker materials have had to be used, and with increased costs, higher loading speeds (of the items to be packaged) have been attempted. For instance the makers of certain potato chips and corn chips and the like have found if the food inside the plastic package is exposed to ultraviolet light, or himidity, then food spoils and their customers are dissatisfied (or if ill, lawsuits sometimes follow). Accordingly, the food packaging industry has move to a package made up of material comprising a lamination of metal foil, paper and polyolefin webs or combinations of derivatives thereof. This new packaging material is thicker than the plastics used heretofore and this thicker material is more difficult to heat and seal into suitable side sealed seams.
The solution does not lie in increasing the heat because above 330.degree. F. the polyolefins suffer thermal degradation and the package is destroyed. In addition, the new packaging material has become costly and increased speeds for packaging is desirable to offset the increased costs by increased production. It has been found that if the temperatures are held to less than 300.degree. F. to prevent thermal degradation, machine speed is limited by heat transfer rates or there results "skips", which are areas or sections along the seam that come apart. The present invention addresses itself to the problem of improving the side seam seal while maintaining an acceptable temperature and accommodating higher throughput speeds.